US1837770A - Process for making electrodes - Google Patents

Description

-Dec.22, 1931. H. N. GILBERT Y 1,837,770

PROCESS FOR MAKING ELECTRODES Filed June 1, 1928 3' 1/ 1o /2 ,fi

INVENTOR.

BY ma ATTORNEY trical conductivities.

Patented Dec. 22, 1931 entree stares raranrorricr.

HARVEY N. GILBERT, OF NIAGARA FALLS,

NEW YORK, ASSXGNOR, BY MESNE ASSIGN- MENTS', TO THE ROESSLER & HASSLACHEB- CHEMICAL COMPANY, OF NEW YORK,

N. Y., A CORPORATION OF DELAWARE PROCESS FOR MAKING ELECTRODES Application filed June 1, 1928. Serial No. 282,257.

This invention relates to improvements in the art of making formed graphite articles and applies more particularly to the method of preparing graphite binder mixtures ready for pressing in the manufacture of electrodes remainder comminuted binder of suitable properties, for example a hard grade of pitch, are mixed and then compressed in a steel mold at a suitable temperature. The mold and contents are then heated to a temperature at which the binder or the pitch is decomposed, The gases produced are permittedto leave the mold but the plastic mixture is prevented from expansion by means of mechanical re-v straining force. After the binder has been decomposed the resulting article can be heated to any higher temperatures required to effect complete baking or even graphitization without deformation or expansion or crack- 'ng.

l have found this process, as claimed, to be fundamental in the art of making composite graphite-carbon articles, e. g, electrodes, of apparent densities over 1. 6 and having great mechanical strength and high elec- This process is not restricted to any particular method of mixing the powdered graphiteand binder and of heating the mixture to the required temperature for pressing but rather its novelty resides in the discovery that in order to secure articles of the desired densities it is neC- essary to prevent expansion or distortion of the mixture after it is compressed particularly during that part of the baking period in which the binder is converted into a rigid mass.

Heretofore in carrying out this process the practice has been to place the mixture of powdered graphite and binder into the mold and to heat the mold until the mixture reached the required pressing temperature. This practice was quite satisfactory in the preparation of small electrodes and was feasible in the preparation of large pieces. However, especially in the latter case, e. g. with pieces having a diameter greater than 8 or 10 inches, this particular method of getting the mixture ready for pressing involves certain inconveniences and difficulties.

The graphite-binder mixture before press ing is a relatively poor heat conductor. Therefore it requires much time or a high temperature gradient to heat the contents of the mold to the pressing temperaturewhen large pi'ecesare made. But this is ociectionable because the binder in the portions of the mixture adjacent to the heated walls often undergoes excessive decomposition on long heating at the pressing temperature and this means loss of plasticity and binding elliciency. Cracking or decomposing the binder to an appreciable extent by overheating prior to pressing must be avoided. I have found that for working on a large scale the heat must be supplied within a sufliciently short 5 time interval and with a suiliciently low temperature gradient to avoid excessive decomposition of the pitch. In accordance with this discovery the mixture should be stirred or agitated during the heating operation and thus save time. Preferably the mixture should be heated outside the mold in mixing vessel and then transferred to the mold for pressing.

Furthermore, in order to produce articles of the required mechanical strength and high apparent specific gravity it wasoften necessary to apply enormous pressures, pressures well over 20,000 pounds per square inch. In

many cases if the pressures were low, say

about 20,000 pounds or less, the resulting articles did not meet the desired specifications. After considerable study and experimentation I have come to the conclusion that the necessity of using such enormous molding pressures when the mixture is preheated to the pressing temperature outside tl e mold in a mixing vessel, or when the stirring and heat-inc of the mixture are done in the mold itself, is due to, or is in some way associated 10G with the fact, that the powdered mixture of graphite and binder occludes or pockets air if prepared in contact with the atmosphere. This absorbed air seems to have a tendency to cause excessive porosity in. the final article and unless very great pressures are applied in compressing the heated mixture the final article does not come up to the desired specifications. 1 1 g It is therefore the principal object of the present invention to provide a method for preparing graphite-binder mixtures ready for pressing under such conditions that the necessity for using excessive pressures be avoided. Other objects will become apparent as the invention is further developed hereinafter. Y

I have discovered that enormous pressures are not necessary in the manufacture of graphite-carbon articles of any commercial size if the mixture of graphite and binder is heated and prepared ready for pressing'm such a way that excessive exposure to and occlusion of air in the mixture and excessive decomposition of the binder is avoided. Indeed, by avoiding occluded air and maintaining in general the preferred conditions for preparing the mixture ready for pressing it is possible to make articles having h1gh apparent specific gravities, 1.7 to 1.9 and higher, with comparatively low pressures such as 3600 pounds persquare inch and even lower pressures, such as 600 pounds per square inch or less, can be used elfectlvely.

All binder that I have thus far found to be suitable for making the graphite-carbon articles of high apparentspecific'gravities of my invention generate some vapors or volatilesduring the heating operation. The presence of these vapors, unless the result of excessive cracking at high temperatures so that hydrogen, methane or similar gases are produced, is not objectionable in the mixture during aressing perhaps because they are relatively easily eondens'able or soluble in the binder under pressing conditions. I have made use of these vapors to sweep out or displace occluded air and to prevent excessive contact with the air by conducting the heat- 2 operation in .a closed vessel which permits egress of gases. Excessive contamination with air must be avoided during all op erations from the beginning of the heating to completion of the pressing. Benzene, xylene or other hydrocarbons or similar substances, relatively easily condensable or soluble under pressing conditions, may be added to the mixing vessel or placed in the mold and vclatilized to aid in displacing non-condensable gases if necessary.

The heating may, if desired, be done in a mold, in which case it maybe necessary to provide a means for agitating the mixture to secure quick heating and to provide for the substantial exclusion of air. It would also be possible to heat the mixture from Within the vessel or mold by inserting heating tubes, heated electrically if desired. In this case agitation may be dispensed with provided no part of the mixture is so far removed from a source of heat as to require too much time to reach the pressing temperature. Provision should be made in all cases for the egress of non-condensable gases but excessive loss of volatile substances from the binder must be avoided. Before pressing the heaters must be removed and the plunger inserted without excessive contact of the mixture with air.

I do not limit myself to any particular method of-heating the mixture for other methods might be used. F or example, the mixture itself, suitably insulated, mi ht be made a resistor to an electric current of the proper voltage and amperage.

In actual practice I prefer to heat the mixture in a mixing vessel out of contact with the air and then to transfer the heated mixture to a mold for. pressing. Provision is made during the transfer from mixer to mold to prevent excessive contact with air.

There are many substances or mixtures of substances that would serve very well as binders for preparing electrode mixtures within my invention and I do not wish to be limited in any way in this respect. It is possible to manipulate pressure, temperature of pressing and percentage of pitch in the mixture so as to secure pieces of suitable density with pitches of different melting points, per cent volatiles, and other characteristics. I have found that I can prepare a good binder by heating a good grade of commercial coal tar pitch in an open vessel up to a temperature of about 350 C. and until the melting point of the pitch reaches between 160 C. and about 210 C. Such a heat treated pitch is called a hard pitch. l/Vide variations in these temperatures are of course permissible and I have obtained fairly good results with pitches melting below 160 C. and above 210 C. By melting point I refer to the temperature at which the corners of chips or small pieces of the pitch be come rounded and wet the surfaces at the point of contact of the capillary tube in which they are heated.

It is also possible to mix the graphite with the untreated commercial pitch and then heat the mixture e. g, in a vessel with agitation, until the pitch in the mixture has acquired the properties necessary for making graphitecarbon articles of the desired apparent specific gravity. The necessary precaution must be observed mixture during the heating operation preceding the pressing, to avoid overheating portions of the mixture and to prevent excessive contan'iination with air on transferring to the mold for pressing. l

to displace occluded air from the V Considerable variation in the optimum pressing temperature is possible and will vary somewhat with the relative degree of hard ness of the pitch. This temperature may be between 250 C. and 450 (3., but I prefer a temperature between 280 C. and 350 C. At temperatures much below 300 C. the binder, perhaps due to excessive viscosity, renders the pressing more or less difficult. In general, however, to secure the best results with this tppe of binder the pressing temperature should be such that the binder is undergoing some decomposition or is producing some volatile vapors. The preferred pressing temperature will in general be at least 50 to 100 C. higher than the melting point of the binder.

However, although it is necessary for best results to heat the mixture before pressing to a temperature at which relatively easily condensable vapors are produced it is n-vertheless essential to avoid excessive loss of volatile matter from the binder. If the loss of volatiles is excessive the melting point of the residual binder will be higher, the displacement of the air from the mixture will be more difficult and the mixture will lose in plasticity.

Variation is permissible in the relative proportions of the binder and the graphite. The larger the proportion of the hinder the more leeway will there be in the pressing temperature and less difficulty with pocketed or occluded air but likewise the final baked article will have a lower apparent specific gravity. On the other hand, the smaller the proportion of binder in the graphite-binder mixture the most important it is to avoid occluded or pocketed air and the less leeway as to the pressing temperature and the greater the possibility of making final articles of high apparent specific gravity. In other words, if high pressures are to be avoided in preparing articles of suitable density with small proportions of binder it is essential to avoid oceluded or pocketed air or othernon-condensable or insoluble gases in the mixture before pressing.

The preferred mixture consists of about- 9.0% of hard pitch and about of graphite. The final baked article will then consist of less than 20% of carbon and more than 20% graphite.

The accompanying drawing illustrates one apparatus that may be used for heating and mixing graphite-binder mixtures. It is of course understood that this invention is in no way limited to this particular apparatus because many other types of enclosed mixing vessels could be employed.

The drawing shows a cross section of the mixer and heater in which 1 is a mixing vessel provided with heat insulation and heated by means of electric heaters 2 with leads 3. l is a shaft operating through the one side of the mixer and having an internal bearing 8, at

the other end. The shaft l is equipped with paddles 7, which sweep the periphery of the mixer. The shaft a is equipped with handle 5 for turning. The entire mixer assembly is mounted on trunnicns 9, which permit it to be rotated to discharge the contents. The chargand discharging of the apparatus is carried out through an opening in the shell, which opening is closed by butterfly valve 10. A. flange, 11 permits connection with the cover 12 which fits over the mold 13 when filling. During the mixing operation when the open ing 1. in the upper position a suitable liquid or mechanical seal can be maintained by connecting suitable listings, not shown, to the flange 11, which permits air and vapors to escape from the mixer and prevents air from entering.

In applying this apparatus to the preparation of electrodes a suflicient quantity of graphite-pitch mixture, e. g., consisting of 80% graphite, 200 mesh and 20% of hard pitchground to approximately mesh, are placed in the n ixer. The outlet of the mixer is connected with a suitable liquid seal which permits air and vapors to escape from the mixer and prevents air from getting back into it. Heat is applied while a slow agitation is maintained until the proper pressing temperature is reached.

Heating, if necessary, is continued until as much of the pocketed relatively non-condensable gas has been displaced by the vapors from the binder as is necessary. After this vapor has swept out the air or non-condensable gas, which may be ascertained by testing by known methods, the heating is stopped and the mixer is turned through so that the opening faces downward. The butterfly valve 10 being closed, the cover which fits on the mold is connected to the outlet from the mixer. The butterfly valve is then opened and the mixture allowed to fall rapidly into the mold which has been previously heated to the same temperature as the mixture.

In some cases it is desirable to have an atmosphere of relatively easily condensable vapor such as benzol'vapor in the mold before the mixture is discharged into it and this can be done, for example, by dropping liquid benzol into the heated mold just before the mixture is transferred into it. In other cases it has been found desirable to make a tight connection between the mold and the mixer and to apply a partial vacuum to the mold just before transferring, but in any event the procedure is so conducted that a minimum exposure to air is experienced in dropping the mixture into the mold.

After transferring the mixture into the mold the latter is placed in a hydraulic press and the desired pressure applied to the mixture. From this point on the procedure is exactly the same as that outlined in my earlier application mentioned above, in which the pressed material in the mold is prevented from expanding until the baking has proceeded suhiciently far to convert the binder into a rigid mass. The baked article may be subj ected to any desired additional heat treatment and may be heated to a graphitizing temperature it it is desired to produce an article of substantially 100% graphite.

It may be desirable, under certain conditions to remove the compressed mixture from the mold before carbonizing the binder. It is then necessary to cool the mold and the contents to a temperature well below the softening point of the pitch or binder before removing from the mold otherwise the compressed piece may be deformed, expand and develop fissures or cracks. pieces it taken out of the mold at a suitable temperature may be stored for an indefinite length of time but it is of course necessary to place them in suitable restraining molds before subjecting them to baking temperatures.

Claims:

1. In the manufacture of graphite-carbon electrodes the step which comprises displacing air or other relatively non-condensable gases from the electrode mixture, prior to molding and baking, by volatilizing a relatively easily condensable material within said nixture while maintaining said mixture substantially out of contact with air. 7 52. Process of preparing an electrode' m ix ture for molding Whichcomprises mixing comminuted graphite with a binding material containing an easily volatilized hydrocarbon and substantially heating this mixture out of contact with air so as to displace air and other non-condensable gases in the mixture by vapors of said hydrocarbon.

3. Method of producing a molded body suitable for carbonization to form an electrode body which comprises forming a mixture of comminuted graphite and a binder containing a small amount of relatively easily volatilizable constituents, heating this mixture to volatilize a portion of said volatilizable material and sweep out and substantially replace air or other relatively non-conden sable gases in said mixture by the volatilized matter, placing said mixture in a mold with substantial exclusion of air and finally coinpressing the material in the mold.

4. Method of producing a moldedbody suitable for carbonization to form an electrode body which comprises forming a mixture of comminuted graphite and a binder containing a small amount of relatively easily volatilizable constituents, heating this mixture to volatilize a portion 01 said volatilizable material and sweep out and substantially replace air or other relatively noncondensable gases in said mixture'by the volatilized matter, substantially replacing the air in a mold by a volatilized hydrocar- Such formed bon and subsequently hot molding the prepared mixture in said prepared mold with substantial exclusion of air and compressing said material in said mold.

Method of producing a molded body suitable for carbonization to form an elec trode body which comprises forming a mixture of comminuted graphite and'a binder containing a small amount of relatively easily volatilizable constituents, heating this mixture to volatilize a portion of said volatifirable material and sweep out and substantially replace air or other non-condenseble gases in said mixture by the volatilized in iai, placing said heated mixture in a mo; l with substantial exclusion of air, subjecting the material in the mold to a vacuum and finally compressing said mixture in the mold.

6. Process which comprises subjecting amixture consisting of upwards of of graphi e and the remainder hard pitch low in volatile matter to agitation while heating to a temperature above the melting point of the pitch at which the pitch produces vapors, and continuing to heat and agitate until substantially all air has been sw pt out of the mixture and container by said vapors, thereafter transferring said mixture to a mold with substantial exclusion of air, pressing and baking to a rigid mass while preexpansion of the pressed article by iaiiical means.

1'. Process which comprises mixing comminute/t graphite and pitch binder in such pro-portions as to produce a final baked article containing more than 80% of graphite, heating the mixture to a pressing temperature under such conditions that occluded relatively.non-condensable gases are replaced by vapors which do not produce gas pockets under dressing conditions, pressing the mixture and thereafter heating to a temperature to convert the binder into a rigid mass while preventing ex ansion of the pressed article by mechanical means.

8. In the process for producing a shaped ;;=hitecarbcn article having an apparent specific gravity ov r 1.6 nd containing upwards or 80% gra ite, the teps which comprise mixing powdered graphite with a carbonaceous binder, heating the mixture by ap plying heat within a suiiiciently short time inte al and a sufficiently low temperature gradient to avoid excessive decomposition of the binder to a temperature at which vapors are generated in the mixture, said vapors displacing the air from the mixture, compressing the heated mixture before the bindor has undergone excessive decomposition.

mixture until the binder has become a. rigid mass.

9. In the process forproducing a shaped graphite-carbon article having an apparent specific gravity over 1.6 and containing upwards of 80% graphite the steps which comprise mixing powdered graphite with a carbonaceous binder heating the mixture with agitation to a temperature between 250 C.- 450 C, displacing the air from the mixture with condensable vapors, compressing the heated mixture before the binder has undergone excessive decomposition in a mold by applying a pressure between about 600 and 3600 pounds per square inch,preventing noncondensable. gases from displacing said vapors from said mixture during and between said heating and pressing, and baking the compressed mixture at a carbonizing temperature the while preventing expansion of the mixture until the binder has become a rigid mass.

10. In the process for producing a shaped graphite article havingan apparent specific gravity over 1.6 the steps which comprise mixing powdered graphite with a carbonaceous binder, heating-the mixture by applying heat within a sufiiciently short time int rval and a suiiiciently low temperature gradient to avoid excessive decomposition of the binder to a temperature between 250 Gr" -Ll (l, displacing the air from the mixture ith condensable vapors, compressing the heated mixture before. the binder has undergone excessive decomposition in a mold by applying a pressure less than 20,000 pounds per square inch, preventing non-condensable gases from displacing the said vapors from said mixture during and between said heating and pressing, and baking the compressed mixture at a carbonizing temperature the while preventing expansion of the. mixture until the binder has become a rigid mass.

11. in the process for producing a shaped graphite-carbon article having an apparent specific g ivitv over 1.6 and containing upwards of graphite the steps which comprise mixing powdered graphite with a carbo aceous binder, heating the mixture with ag ation to a t mpe 'ature between 280 C. and 350 (1, displacing the air from the mixture with condensable vapors, compressing the heated mixture before the binder has undergone excessive decomposition in a mold by applying a pressure between about 600 and 3500 po nos per square inch, preventing non condez Mble c ses from displacing said vapors from saio. mixture during and between said heating and pressing, and baking the compressed mixture at a carbonizing temperature the whi e preventin expansion of the mix ure until the binder has become a rigid mass.

12. In the process for producing a shaped graphite-carbon article havin an apparent specific gravity over 1.6 and containing upwards of 80% graphite the steps which comprise mixing powdered graphite with a carbonaceous binder, heating the mixture with agitation to a temperature at least 50 C. above the melting point of the binder, displacing the air from the mixture with condensable vapors, compressing the heated mixture before the binder has undergone excessive decomposition, preventing non-condensable gases from displacing the vapors from said mixture during and between said heating and pressing and baking the compressed mixture at a carbonizing temperature the while preventing expansion of the mixture until the binder has become a rigid mass.

13. In the process for producing a shaped graphite article having an apparent specific gravity over 1.6 the steps which comprise mixing powdered graphite with a carbonaceous binder, heating the mixture by applying heat within a sufliciently short time interval and a sufiiciently low temperature gradient to avoid excessive decomposition of the binder to a temperature between 280 C. and 350 (3., displacing the air from the mixture with condensable vapors, compressing the heated mixture before the binder has undergone excessive decomposition in a mold by applying a pressure less than 20,000 pounds per square inch, preventing non-condensable gases from displacing the said vapors from said mixture during and between said heating and pressing, and baking the compressed mixture at a carbonizing temperature the while preventing expansion of the mixture until the binder has become a rigid mass and there after-subjecting the baked article to further heating to graphitize the carbon.

i l. In the process for producing a shaped graphite article having an apparent specific gravity over 1.6 the steps which comprise mixing powdered graphite with a carbonaceousv binder, heating the mixture with agitation to a temperature at which vapors are generated in the mixture, said vapors displacing the air from the mixture, compressing the heated mixture before the binder has undergone excessive decomposition, preventing air from displacing the vapors from said mixture during and between said heating and pressing, and baking the compressed mixture at a c'arbonizing temperature the while preventing expansion of the mixture until the binder has become a rigid mass, and thereafter subjecting the baked article to further heating to graphitize the carbon.

15. In the process for producing a shaped grmohite-carbon article having an apparent specific gravity over 1.6 and containing upwards of 80% graphite the steps which comprise mixing powdered graphite with hard pitch, heating the mixture by applying heat within a sufliciently short time interval and asufiiciently low temperature gradient to aroid excessive decomposition of the pitch to temperature' at which vapors are generated in the mixture, said vapors displacing the air from the mixture, compressing the he I ted mixture before the pitch has undergone Live decomposition, preventing air from said vapors from said mixture uni-ing d betw-een'said heating and press ing, and baking the compressed mixture at a carbonizing temperature the while preventing expansion of the mixture until the binder has hecome a rigid mass.

16. In the process for producing a shaped graphite-carbon article having an apparent gravity over 1.6 and containing upwards of graphite the steps which comprise mixing powdered graphite with hard pitch, heating the mixture with agitation to a temperature between 280 C. and 350 0., displacv f from the mixture with condenin a mold before the pitch has undergone excessive decomposition by applyin a pressure less than 20,000 pounds per square inch, preventing air from displacing said vapors from said mixture during and between saic heating and pressing, and baking the compressed mixture at a carbonizing temperature the while preventing expansion of the mixture until the binder has become a rigid mass. i

17. In the process for producing a shaped graphite article having an apparent specific gravity over 1.6 the steps which comprise mixing powdered graphite with hard pitch, heating the mixture with agitation to a temperature between 280 C. and 350 (5., displacing the air. from the mixture with condensable vapors, compressing the heated mixture before the pitch has undergone excessive decomposition, preventing air from displacing said vapors from said mixture during and between said heating and pressing, baking the compressed mixture at a carbonizing temperature the while preventing the expansion of the mixtureuntil the binder has become a rigid mass, and thereafter subjecting the baked article to further heating to graphitize the carbon.

18. In the process for producing a shaped graphite-carbon article having an apparent specific gravity'over 1.6 and containing upwards of 80% graphite the steps which comprise mixing the powdered graphite with hard pitch, heating the mixture with agitation to a temperature at least 50 C. above the melting point of the pitch,displacing the air from the mixture with condensable vapors, compressing the heated mixture in a mold before the pitch has undergone exces-- sive decomposition by applying a pressure square inch preventing air from displacing said vapors from sa d mixture during and a sufiiciently short time interval and a sufficiently low temperature gradient to avoid excessive decomposition of the pitch to a temperature between 280 C. and 350 C.,

displacing the air from the mixture with condensable vapors, compressing the heated mixture in a mold before the pitch has undergone excessive decomposition by applying a pressure between about 600 and 3600 pounds per square inch, preventing non-condensable gases from displacing said vapors from said mixture during and between said heating and pressing, baking the compressed mixture at a carbonizing temperature the while preventing expansion of the mixture until the binder has become a rigid mass, and thereafter subjecting the baked article to further heating to graphitize the carbon:

20. In the process for producing a shaped graphite-carbon article having an apparent specific gravity over 1.6 and containing upwards of 80% graphite the steps which coinprise mixing powdered graphite with hard pitch, heating the mixture to a temperature at least 50 0. above the melting point of the pitch by applying heat within a sufiiciently short time interval and a sutiiciently low temperature gradient to avoid excessive decomposition of the pitch, displacing the air from the mixture with condensable vapors, coin;

pressing the heated mixture before the pitch has undergone excessive decomposition, preventing air from displacing the vapors from said mixture during and between said heating and pressing, and baking the compressed between about 600 and 3600 pounds per CERTIFICATE OF CORRECTION.

Patent No. 1,837,770. Granted December 22, 1931, to

HARVEY N. GILBERT.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, line 35, for "binder" read binders; page 3, line 52, for "20%" read 80%; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 9th day of February, A. D. 1932.

M, J. Moore, (Seal) Acting Commissioner of Patents.

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